1. Technical Field
Embodiments relate to an elementary image acquisition or display device, capable of being used in integrated image sensors or liquid crystal display projectors.
2. Discussion of the Related Art
FIG. 1 illustrates a front-side illuminated image sensor, comprising a pixel array formed at the surface of a semiconductor substrate 10.
Each pixel P1, P2, P3 comprises an area where photogenerated charges 12 are collected and active elements for transferring the collected charges 14 to an adapted electronic circuit. An interconnection stack 16, comprising conductive tracks and vias 18 separated by an insulating material 20, is formed above substrate 10 to connect the elements for transferring the collected charges to the rest of the electronic circuit of the image sensor. Preferably, the conductive tracks and vias extend in front of the periphery of the collection areas to avoid getting in the way of the light rays. Each pixel is insulated from the neighboring pixels by buried insulating regions 22 formed in substrate 10.
To improve the detection of incident light beams, it may be provided to direct these beams towards collection areas 12 by means of a microlens 24 defined in front of each of the pixels, on interconnection stack 16. This enables limiting parasitic reflections of incident photons on the conductive regions of the interconnection stack.
FIG. 2 illustrates a disadvantage associated with this type of structure.
In FIG. 2, the pixel has, at the surface of substrate 10, dimensions greater than the thickness of interconnection stack 16. A microlens 24, formed at the pixel surface, has a radius of curvature which is not sufficient to focus all incident light beams towards the center of the photodetection area. As a result, beams 26 reaching the periphery of the microlens are focused towards conductive tracks of the interconnection stack, instead of being focused towards substrate 10. Beams 26 are reflected by the conductive tracks of the stack and are not collected, which adversely affects the image sensor detection.
To solve this problem, it could be envisaged to form a thicker microlens, which would thus be more rounded. However, microlens manufacturing methods have their limits, thus imposing a maximum lens thickness, which is not adapted for large pixels such as the pixel of FIG. 2. Further, very thick microlenses tend to cause spherical aberrations and thus to adversely affect the detection.
Conventional microlens structures associated with image sensors are thus not ideal for pixels having dimensions (the “pitch”) close to (of the same order of magnitude) or greater than the thickness of the interconnection stack formed at the substrate surface. More specifically, such microlens structures become unsuitable when the pixel size becomes greater than 0.8 times the thickness of the interconnection stack (for example, for pixels having substrate surface area dimensions greater than 3 μm).
For example, for an interconnection stack thickness on the order of 3 μm, such microlens structures become unsuitable for pixels with pitches greater than 2.4 μm.
There thus is a need for a focusing structure adapted to large-pitch pixels such as discussed hereabove.